1. Field of the Invention
This invention relates to large superconducting energy storage magnets and particularly to a modular construction of such magnets with shorting switches for emergency dissipation of energy.
2. Background Information
Superconducting magnetic energy storage (SMES) inductors or magnets are very large inductors made of superconducting materials which are capable of storing large amounts of energy and/or generating large magnetic fields SMES inductors are very efficient for these purposes because no energy is lost to resistive heating in the superconducting current path. SMES inductors are operated at cryogenic temperatures. If a section of the inductor goes normal (exits the superconducting state), that section will rapidly overheat unless protective measures are taken. One well known protective procedure is to drive the entire inductor normal, so that the energy is dissipated evenly throughout the inductor.
SMES inductors are hundreds or thousands of feet in diameter. Typically, they are large solenoids. Presently designed implementations have two or four layers of turns. Each layer extends the full height of the inductor and is tens of feet high, but only inches wide. These inductors are bath cooled by immersing the superconducting path in a pool of coolant, such as liquid helium. The dewar, which is the vessel holding the coolant, is tens of feet high, but only several feet wide. Commonly, for ease of construction, the dewar is divided into sections with transverse plates between, although these sections are usually interconnected to form one heat transfer unit. Since each layer of turns extends the full height of the inductor, each must penetrate the plates between sections of the dewar.
The layers of the SMES inductor are connected in series or parallel, or some combination of series and parallel. If adjacent ends of two layers are connected together so that the current flows upward in one layer and downward in the other, the layers must be wound in opposite directions so that the magnetic flux produced by the two layers is in the same direction. While this arrangement has the advantage of short connections between the layers, it produces higher internal voltages between layers. Alternatively, layers can be connected in series by cross connections which connect opposite ends of adjacent layers together so that the current flows in the same direction through both coils and therefore both coils are wound in the same direction. This has the advantage of reducing layer to layer voltages but requires very long cross connectors.
With the present technological state of the art, SMES inductors have inherent limitations on their terminal voltages and currents. If more power must be extracted/inserted into an inductor than is practical through a single set of leads, the layers can be connected to provide two parallel circuits. A typical arrangement in a four layer inductor is to have two parallel circuits each having two layers connected in series.
When a problem occurs in the SMES inductor, the coolant is drained out of the dewar over a period of time, such as 30 to 40 seconds. As the liquid coolant level drops below turns, the uncovered turns become normal. With the draining of the liquid coolant from the dewar, the SMES coil doubles as a dump resistor. That is, the coil is an inductor which stores energy, but also becomes a resistor which dissipates the energy. The process of draining the liquid coolant and dissipating the coil energy is known as a protection dump. Although the coil terminals are grounded during a protection dump, voltages to ground are generated internal to the inductor during this procedure. This arises because the resistance and inductance are not distributed equally per unit length of the inductor. The turns at the center of the inductor have the highest total inductance while the turns on the top of the inductor have the highest resistance. The turns at the top of the inductor have the highest resistance because resistance is a function of temperature, and the top begins to heat first as the coolant level drops. To manage the voltages that develop internal to the coil during a protection dump, it has been proposed that shorting switches be used to short each layer of the inductor periodically along its length. Thus, the number of shorting switches required is equal to the number of switches per layer multiplied by the number of layers. For instance, in a four layer inductor in which each layer is shorted by four shorting switches, a total of sixteen shorting switches are required. All sixteen shorting switches must operate to reduce the internal voltages to the desired levels.
It is an object of the present invention to provide an improved SMES inductor.
More particularly, it is object of the invention to provide such an improved SMES inductor in which a protection dump can be performed with fewer shorting switches than presently possible.
It is another object of the invention to provide such an improved inductor with fewer penetrations required between dewar sections.
It is also an object of the invention to provide such an inductor in which sections of the inductor can be taken out of service easily.
It is another object of the invention to provide such an inductor which can be easily scaled to any size required.
It is yet another object of the invention to provide such an inductor which can be easily and economically fabricated.